A ceramide is a sphingolipid composed of sphingosine and a fatty acid linked to sphingosine or phytosphingosine. A family of ceramides, which make up about 40% or more of the lipids present in cornified cells, is essential in controlling the structural organization and functions of the stratum corneum, among other vital physiological functions thereof.
Ceramides protect the skin from internal/external stresses, e.g., by removing disrupted cells, and they become depleted with advancing age. As a consequence, a number of detrimental factors such as the exposure to the external stimuli such as ultraviolet ray and chemicals cause water loss and the exfoliation of cornified cells, which causes the skin to become dry, cracked and fissured.
Such symptoms may be partially ameliorated by external application of ceramides. However, if a large amount of insoluble stratum corneum components are used in a cosmetic formulation, the amounts of additives such as emulsifiers and oils must be increased to enhance the solubility of the corneum components, which, however, results in the difficulty of forming a structure similar to the stratum corneum. Further, ceramides tend to aggregate and form a gel unsuitable for a cosmetic formulation, and therefore, its amount is generally limited to less than 1% by weight.